In a typical cellular radio system, wireless terminals, also known as mobile stations and/or user equipment nodes (UEs), communicate via a radio access network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment nodes (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment nodes (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity.
Specifications for the Evolved Packet System (EPS) have completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to radio network controller (RNC) nodes. In general, in E-UTRAN/LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes, e.g., eNodeBs in LTE, and the core network. As such, the radio access network (RAN) of an EPS system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
FIG. 3 shows, in simplified manner, example architecture of the 3G Long Term Evolution (LTE) system. In the above as mentioned above, LTE is based on a flat architecture compared to 2G and 3G systems. Each cell is served by an eNodeB or eNB (“base station”), and handovers between cells can be handled either via the Mobility Management Entity (MME) and the S1 interface, or directly between the eNBs via the X2 interface.
The current solutions existing for interference coordination and mitigation have been designed for intra-carrier operation. Namely, these solutions will help reducing interference within a carrier already in use by a number of cells in a given neighbourhood.
The solutions so far standardised rely on signalling over the X2 interface of the following parameters:                A Relative Narrowband Transmit Power (RNTP)—used to give an intra-carrier indication of DL interference;        a High Interference Indicator (HII)—used to give an intra-carrier indication of UL interference sensitive PRBs; and        an Overload Indicator (OI)—used to provide an intra-carrier indication of overall UL interference.        
In the tenth release of LTE (e.g., rel10) an extra mechanism to reduce interference has been finalized by the use of Almost Blank Subframes (ABS). ABS involves signalling, over the X2 interface, a pattern of intra-carrier subframes where the aggressor will reduce its transmissions to allow the victim to have interference free communication.
What has thus far not being designed or standardised is a mechanism that relies on inter-carrier solutions for interference mitigation. Such mechanism could be of relevance for operators with more than one carrier available and with the possibility/willingness to let certain types of cells to freely use a carrier within a certain carrier range in order to minimise overall interference.
A recent 3GPP contribution (3GPP TSG-RAN WB3 Meeting #72, R3-111419, Barcelona, Spain, May 9-13, 2011, Agenda item 14.1, “A Plan for LTE Rel-11 Carrier Based Interference Management”, incorporated herein by reference) outlined the general need of inter-carrier interference mitigation solutions but did not describe any structural or operational details.
What is needed therefore, and thus an object of at least some of the technology disclosed herein, are apparatus, methods, and techniques for carrier based interference coordination/mitigation.